Modify async to async_ to support Python 3.7

[mmilk1231]

async becomes a keyword in Python 3.7. Thus when I import ChainerRL with Python 3.7, the following SyntaxError happened:

  File "/usr/local/lib/python3.7/site-packages/chainerrl-0.3.0-py3.7.egg/chainerrl/agents/a3c.py", line 122
    async.assert_params_not_shared(self.shared_model, self.model)
         ^
SyntaxError: invalid syntax
...

This PR avoid the error by modifying async to async_.

References about Python 3.7:
https://www.python.org/dev/peps/pep-0492/
https://docs.python.org/3/whatsnew/3.7.html#changes-in-python-behavior

[toslunar]

Could you break this line (>79 chars)?

[mmilk1231]

I'm sorry for my mistakes. I break this line in the new commit.

[toslunar]

Thanks a lot for sending the PR. This fixes #286. The new name async_ looks good to me.

Could you also fix imports in tests? (chainerrl/tests/misc_tests/test_async.py)

[mmilk1231]

I fixed chainerrl/tests/misc_tests/test_async.py by rewriting async to async_.
Thank you for pointing it out.

[toslunar]

LGTM